This invention relates to a calibration device, a calibration method and a method for manufacturing a calibration device.
Plastic profiles of thermoplastic materials in particular are manufactured by means of extrusion. For shaping a plastic profile a nozzle and a so-called calibration unit are used.
Calibration units for the manufacture of plastic profiles, e.g. window profiles, usually include a dry and a wet calibration device, wherein after exit from the nozzle the hot extrudate first passes through the dry calibration device and subsequently through the wet calibration device. At the end of the calibration and after complete cooling of the plastic profile to room temperature, the plastic profile must have the desired contour.
The wet calibration device includes e.g. several vacuum tanks arranged in series for largely cooling the plastic profile, after the same has already partly been cooled in the dry calibration device. In the wet calibration device, calibrating elements (so-called short calibrators or covers) are arranged at intervals from about 100 mm on the inlet side to about 500 mm on the outlet side, which each have a through opening which largely corresponds to the geometry of the plastic profile.
The vacuum tanks in principle are closed on all sides and only at the end faces have openings for the entry and exit of the plastic profile. The vacuum tanks are charged with negative pressure in an order of magnitude about 30 to 200 mbar, whereby the plastic profile is slightly “bloated” and attaches to the through openings of the calibrating elements.
The plastic profile thus cools down in a defined shape, so that the dimensional stability is ensured. The cooling water flows through the vacuum tanks proceeding from few supply lines down to few discharge lines. The negative pressure in the vacuum tanks only must be applied via a negative pressure port and then acts in all vacuum tank interior spaces.
The manufacturing effort for the calibrating elements is comparatively low, because the same do not require any supply and distribution bores for negative pressure and cooling water.
The dry calibration device upstream of the wet calibration device includes up to eight calibrating elements. These calibrating elements each include running surfaces which approximately correspond to the contour of the plastic profile. In detail, however, dimensional or geometrical differences do exist, in order to follow the reduction in length with decreasing temperature of the plastic and in order to compensate the distortion as a result of different cooling rates.
In particular in the region of the vacuum tanks, the plastic profile has direct contact with the cooling water. Particles from the cooling water, e.g. sand or lime particles, can be deposited on the sensitive running surfaces of the calibrating elements or on their inlet edge, whereby scratches then are produced on the plastic profile surface. Such scratches chiefly are disturbing at the visible surfaces of plastic profiles. These are those surfaces of the plastic profiles which e.g. in the completely mounted window are oriented inwards into the room or outwards into the open air.
Experience shows that the cooling water frequently is contaminated by organic and/or inorganic impurities (e.g. particles) as a result of the comparatively long duration of use in the extrusion system. This is apparent from the fact that the cooling water mostly is slightly to strongly turbid.
The patent application DE 10 2012 218 140.7 dated Apr. 10, 2012 of the same applicant discloses that the sensitive visible surfaces of a profile in the vacuum tank can be supported by guide elements in the form of rollers. In so far, reference is made herewith expressly to DE 10 2012 218 140.7.
Dirt particles from the cooling water, which get into the gap between roller and profile surface and are deposited there, therefore cannot produce any long scratches, but at best a unique impression. After “rolling through” the gap, the particles then generally again remain in the cooling water.
Since the rollers are not driven or because a drive would be very expensive, it is very important that the rollers run smoothly, so that they do not stand still and then all the more lead to scratches. The rollers therefore must have a rather low-friction bearing which in the given environment, such as more or less polluted water and very cramped space conditions, prevents stopping of the rollers as reliably as possible and which e.g. also allows an exact positioning of the roller.